Drop forging under screw presses requires a comparatively large amount of energy and therewith a heavy flywheel. To protect the press from inadmissable overstressing because of a rebound blow, it is known to couple the press spindle using a spring-loaded slipping coupling. Surplus energy is then eliminated through friction work in the slipping coupling.
The control device described in German Patent Specification No. 28 37 253 utilizes the slipping at the time of the blow of the screw press. Such slipping develops in the coupling as the coupling torque is exceeded, that is, the resultant relative motion between flywheel and spindle, in order to shift the coupling. In this known structure, a control lug is in the form of an oblong swivel lug and comes into contact with the peripheral surface of the coupling portion fixed with respect to the spindle. Such contact comes when the coupling portion and the flywheel run synchronously while pressure medium acts upon the respective associated piston.
Slipping of the coupling portions sets in under load when the speed of the coupling portion on the side of the spindle is reduced with respect to the speed of the flywheel. When such slipping occurs in this known structure, the control lug swings out of the engaged position so that the piston of the control lug is pushed further in the direction of the center of the press. The control lug moves under the working pressure of the pressure medium present in the first instance. Upon control lug movement there is a rapid fall in pressure in the cylinder chambers and in the coupling pressure chamber connected therewith. Consequently, uncoupling takes place due to the coupling return-springs. The action of the pressure medium follows pneumatically.
In the case of controlled reactions of this kind, it follows that the throwout of the coupling is only initiated at that point in time at which the transferable torque has already been exceeded. Before the reaction of the coupling for the reduction of the torque, more time passes in which necessary friction is carried out thereby resulting in wear and heating with a deterioration in the level of efficiency. Basically, it is too late to shift only when the coupling is overloaded.
Other known shifting mechanisms are dependent upon angular deceleration and utilize the inertia forces of reaction masses arranged on the spindle. German Patent Specification No. 28 01 139 discloses a screw press equipped with a pneumatically operated coupling having a pneumatic rapid vent valve. When a disk is assigned to the spindle as a movable mass, the disk moves as a result of the deceleration of the spindle at the time of the press blow. Such disk movement is affected by a toothed surface axially to the spindle and causes therewith rapid venting of the coupling via anticipatory and main control. Use of compressed air is disadvantageous because it requires a comparatively long expansion time. Indeed, if the coupling is adjusted with different levels of air pressure to achieve different torques and therewith different powers of impact, the shifting member, which functions with inertia, is excluded from this adjustability and only reacts when angular deceleration of the spindle is constant at all times.
The screw press described in German Patent Specification No. 26 43 534 includes a shifting mechanism that is dependent upon angular deceleration and utilizes the inertia forces of the reaction masses arranged on the flywheel. This coupling device has a spring link joint which is dependent upon the speed of the work spindle which comes into operation when a given pressing action is achieved and which is developed as a quick-break shifter. A control disk flywheel as a reaction mass exerts a force upon the spring link joint, whereby the work spindle is uncoupled from the flywheel. In this known construction, reaction movements which are much too great are required at the control mechanism for the actuation of a coupling. Thus, the reaction time is clearly prolonged and this is detrimental to the whole shifting process.